Peel and finely chop the onion. Trim and finely chop the celery. Warm a large pan over a low heat for 1 min. Add 1 tbsp olive oil, the onion and celery. Season with salt and pepper. Cover and sweat for 10 mins.

While the veg sweat, slice the roots off the celeriac at a diagonal, then peel the rest of the celeriac. Roughly chop it. Peel, core and chop the apple. Stir both of them into the pan. Add 1 bay leaf and most of the rosemary sprigs.

Crumble in a stock cube, then pour in 850ml boiling water. Cover, turn the heat up and bring to the boil. Turn the heat down a little and simmer for 20 mins till the veg are very tender.

Fish the bay and woody rosemary sprigs out of the soup (don't worry if a few leaves are left behind). Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste and add more salt and pepper if you think it needs it. Serve garnished with a few fresh rosemary leaves.